| The goat milk coagulation ability is poor,after processing yoghurt will lead to yoghurt whey separation and cheese yield decreased.Ultrasonic treatment is a non-thermal processing technology,widely used in food processing industry,can effectively play the role of sterilization,homogenization,emulsification,modification et al.In recent years,ultrasonic processing technology has become a research highlight for improving the coagulation ability of goat milk.The effects of low-intensity(150-400 W)and high-intensity(2000-4000 W)ultrasound treatments on milk physicochemical,microbial and protein structure of goat milk were carried out.Ultrasound on the rheological properties and microstructure of goat milk rennet induced gel were studied.The effects of ultrasound treatment on improving stability and sensory quality of goat yoghurt during storage were further evaluated.The results were as follows:Thermosonication was used as a combined treatment of raw goat milk(RGM)using pasteurization(72oC for 15 s)and ultrasound treatments(20 kHz at the power variance of 150 W,200 W,300 W and 400W for 10 min).Application of thermosonication on goat milk revealed a reduction in microbial load,viscosity,turbidity and particle size of fat and casein micelles without any effect on colloidal calcium phosphate(CCP)and the native protein.However,no significant effect observed on the native state of casein micelles,pH values,minerals balance,and the colour properties.While,goat milk was treated by ultrasound(US)at 20 kHz and 12.1±0.89 W(output power 800W)from(15 to 30 min)reduced the particle size,increased hydrophobicity,and changed the secondary structures of the protein which were significantly increase in?-sheets and random coils.While at 4000 W,Thermosonication altered the secondary structure of the protein which decreased?-sheet content and increased the amount of?-helix structure leading to increase the hydrophobicity.As well as in thermosonication milk samples,the percentage of?-sheet structures inversely correlated with the exposure of hydrophobic regions of the protein(r=0.79;r=0.84),While,the percentage of?-helix structure positively correlated to the increase in the surface hydrophobicity(r=0.61;r=0.92)for goat and cow milk samples,respectively.The influence of the treatments on the rheological properties and microstructure of the rennet-induced gel was measured.The US treatments of goat milk were performed at a frequency of 20 kHz and net output power of 800 W at different time intervals(15,20,25,or 30 min)in 5 s:3 s work/rest cycles.Sonication time>20 min,resulted in a significant(p?0.05)increase in the storage modulus(G')compared to the controls(raw goat milk and pasteurized goat milk).While the rennet coagulation time(RCT)was significantly decreased.Scanning electron microscopy(SEM)showed the gel microstructures of the sonicated samples were more interconnected.Meanwhile,thermosonication prior to conventionally heated(95~oC for 10 min)of goat milk(at an ultrasound frequency of 20 kHz and output power 4000 W,from 5 to 15 min)allowed the preparation of yoghurts with microstructure and texture properties superior to those of control yoghurts produced from homogenization milk before heated at the same temperature.Furthermore,thermosonication altered the secondary structure of the protein which decrease?-sheet content and increased the amount of?-helix structure leading to increase the hydrophobicity.Therefore,texture profile and microstructure analysis showed that the yoghurt produced from the thermosonication milk samples had higher firmness,adhesiveness,and deluxe network structure than those induced from the homogenized.In addition to,the effect of thermosonication process pre-treatment(at 20 KHz frequency,and an output power of 4000 W with 25%amplitude for 5,10,and 15 min)on the physicochemical and rheological properties of yoghurt made from goat milk compared to homogenized treatment were studied.The thermosonication process reduced the fat globule diameter lead to induce stirred yoghurt had good quality parameters such as lower syneresis,higher viscosity,and better sensory properties during the storage than the stirred yoghurt of homogenized milk.Confocal scanning laser microscopy demonstrated that the microstructure of thermosonication treated milk yoghurt had more interconnected chains. |
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